An example of the well-known position detectors utilizing magnetostrictive oscillation wave propagated through the magnetostriction propagating media is schematically illustrated by FIG. 13 of the accompanying drawing. The illustrated position detector has been proposed by the applicant of the present application (Japanese Patent Application No. 1983-220071).
In this position detector of the prior art, a plurality of magnetostriction propagating media 1 of an amorphous alloy or the like containing a high percentage of iron each support a first coil 2 wound therearound at their one end and a second coil 4 wound therearound over their length for detection of the positions indicated by the position indicators 3p, 3q. The first coil 2 is connected to a pulse current generator 5 while the second coil 4 is connected to a processor 6. A biasing magnetic field generator 7 comprising a permanent magnet is provided in opposition to the end surfaces of magnetostriction propagating media 1, adjacent the portions of the respective magnetostriction propagating media around which said first coil 2 is wound.
First coil 2 is crossed between each pair of adjacent magnetostriction propagating media 1 and wound around each magnetostriction propagating medium 1 in the reverse direction with respect to the direction in which it is wound around the adjacent magnetostriction propagating medium 1. Each second coil 4 is wound around the respective magnetostriction propagating media 1 in the same direction and serially connected with adjacent second coils so that the connection polarity is successively reversed between adjacent second coils. Such arrangement is effective in reducing the unnecessary magnetic flux emitted external to the system. External noise is cancelled or reduced between said respective adjacent portions or coils because the direction of the magnetic flux generated when the direction of the voltage or the current generated in a coil, when the magnetic flux is reversed, is opposite from one portion or coil to the adjacent portion or coil so that the magnetic flux external to the system is reduced or cancelled.
Application of a pulse current from the pulse current generator 5 to the first coil 2 causes an instantaneous field variation in the first coil 2 and, thereby, a magnetostrictive oscillation wave to be generated in the region of the respective magnetostriction propagating media 1 around which the first coil 2 is wound. This magnetostrictive oscillation wave is propagated through the respective magnetostriction propagating media 1 longitudinally thereof at the propagating velocity specific to the magnetostriction propagating media 1. During this propagation, the conversion from mechanical energy to the corresponding magnetic energy occurs in the region of respective magnetostriction propagating media 1 in which the magnetostrictive oscillation wave exists according to the electromechanical coupling factor (i.e., the factor representing the conversion efficiency from mechanical energy to the corresponding electric energy or from the electric energy to the corresponding mechanical energy) specific to said region and, in consequence, an inductive voltage is generated in the respective second coil 4.
Assuming that a steady magnetic field exists in a region of magnetostriction propagating media 1, which is sufficient to increase the electromechanical coupling factor of this region, high inductive voltages are generated in the second coil 4 in the region when said magnetostrictive oscillation wave reaches said region. With a pair of position indicators 3p, 3q generating such a steady magnetic field, respectively, in two regions, as shown by FIG. 13 two inductive voltage pulses V.sub.1, V.sub.2 corresponding to the positions indicated by the pair of position indicators 3p, 3q are generated, as shown by FIG. 14. Detection of the positions indicated by the respective position indicators is accomplished by detecting the points in time in which the respective inductive voltages V.sub.1, V.sub.2 exceeded a predetermined voltage value (i.e., threshold value E).
Referring to FIG. 14 of the accompanying drawing, the time duration T.sub.1 that elapses between the point in time at which the pulse current is applied to the first coil 2 and the point in time at which the inductive voltage V.sub.1 is detected is substantially equal to the time duration elapsing between the point in time at which the magnetostrictive oscillation wave is generated in the regions of the magnetostriction propagating media 1 around which the first coil 2 is wound to the point in time at which said magnetostrictive oscillation wave reaches the position indicated by the position indicator 3p. Accordingly, this time duration T.sub.1 may be determined and the time lapse so determined may be multiplied by the propagation velocity of the magnetostrictive oscillation wave in the processor 6 to calculate the distance l.sub.1 between the first coil 2 and the position indicator 3p, i.e., the position (coordinates) indicated by the position indicator 3p. The distance 12 between the first coils 2 and the position indicator 3q, i.e., the position (coordinates) indicated by the position indicator 3q may be calculated on the basis of the time lapse T.sub.2 taken before detection of the inductive voltage V.sub.2.
In the position detector illustrated in FIG. 13, the inductive voltages V.sub.1, V.sub.2 derived from the pair of position indicators 3p, 3q are provided as a composite output, since the first coil 2 and the second coil 4 wound around the respective magnetostriction propagating media 1 are connected in electrical series in a single circuit.
As a consequence, the composite inductive voltage may not exceed the threshold value E as best seen in FIG. 15(c) when the difference between said distances l.sub.1 and l.sub.2 is small enough. Specifically, the inductive voltage V.sub.1 derived from the position indicator 3p located at the distance l.sub.1 takes a form as illustrated by FIG. 15(a) while the inductive voltage V.sub.2 derived from the position indicator 3q takes a form as illustrated by FIG. 15(b). The combination of these inductive voltages V.sub.1, V.sub.2 results in a mutual cancellation between the positive component and the negative component thereof, as illustrated in FIG. 15(c). When the distances l.sub.1, l.sub.2 are equal to each other, the resultant detection erroneously suggests that a single position is indicated, or no position is indicated, in spite of the actual situation that two positions are indicated.
In view of the above-mentioned disadvantage, i.e., the combination or composition of the two inductive voltages V.sub.1, V.sub.2 , another type of position detector has been proposed, in which both the first coils 2 and the second coils 4 are wound around the individual magnetostriction propagating media 1 so that the pulse current is separately applied to the respective first coils 2 and thereby detection of said inductive voltages as well as determination of the time lapse before generation of such inductive voltages is also separately achieved. With a position detector of this type, the number of the magnetostriction propagating media increases inconveniently when the position detection is required over a wide range and the time required for determination becomes longer since the determination must be done separately for said increased number of the magnetostriction propagating media.